Chapter 2: Protozoa: Myxomycota (slime moulds)

The chapter presents four major groups of slime moulds, each displaying unique life cycle patterns and developmental mechanisms. Acrasiomycetes represent the simplest cellular slime moulds, where individual amoebae converge into multicellular aggregates called pseudoplasmodia and subsequently develop into sorocarps for spore dispersal. Dictyosteliomycetes, exemplified by Dictyostelium discoideum, demonstrate a remarkable transition toward multicellularity wherein thousands of independent amoebae respond to chemical gradients of cyclic adenosine monophosphate to organize into slug-like structures that migrate and eventually differentiate into specialized stalk and spore cells, raising profound questions about cooperative behavior, evolutionary trade-offs, and cheating mechanisms within cellular populations. Protosteliomycetes form delicate stalked structures from either amoeboid or plasmodial stages, employing diverse reproductive strategies including the production of flagellated dispersal cells. The Myxomycetes represent the largest and most conspicuous group, featuring macroscopic plasmodia that function as multinucleate protoplasmic networks capable of phagocytic feeding and remarkable behavioral responses including maze navigation and network optimization. These organisms produce various fruiting structures including sporangia, aethalia, and plasmodiocarps, with specialized tissues such as capillitia and elaters governing spore release. The chapter emphasizes that despite historical fungal associations, slime moulds employ phagocytosis for nutrition rather than extracellular enzymatic absorption, fundamentally distinguishing them from true fungi while allowing them to occupy similar ecological niches. Physarum polycephalum serves as a primary model organism for investigating synchronized cell division, cytoplasmic streaming regulated by calcium dynamics, and emergent collective intelligence, providing experimental insights into fundamental biological processes including cell signaling, morphogenesis, and the evolution of multicellular organization from unicellular ancestors.